The present invention relates to ice thermal storage units and to the heat exchanger arrangements, such as a cooling coil used to cool and freeze the storage fluid within the storage tank. More specifically, coil arrangements to facilitate ice melting after an overbuild of ice within a thermal storage unit are identified, which arrangement enable maintenance of an adequately low temperature for the thermal storage unit outlet fluid.
Ice thermal storage units provide a means of storing cooling capacity for use at a later time. A frequent application of such thermal-storage equipment utilizes lower cost electrical energy, usually from the evening and night time hours, to generate and store a volume of ice in a large vat or chamber filled with a fluid, such as water. This ice-water mixture is retained until its stored cooling capacity is required, which requirement is usually experienced during high-demand, high-cost periods, such as daytime hours. In a typical operation, the low-temperature fluid is withdrawn from the chamber, pumped through a heat exchanger to absorb heat, and is then returned to the thermal storage unit chamber to be cooled by melting the retained ice. An exemplary application of stored cooling capacity is a district cooling operation, which is becoming a more widely accepted cooling practice in larger communities. These district-cooling operations generally have multiple heat exchangers coupled to a single ice thermal storage facility. The larger number of different users of the thermal storage unit in a district cooling application requires maximum utilization of both physical space and energy.
Unmonitored or improperly controlled ice storage units may overbuild the stored ice. That is, the ice storage chambers most frequently incorporate a plurality of refrigeration coils to cool and freeze the water or other fluid in the chamber. During the storage or build cycle, the fluid is cooled until the ice develops on each tube. As in most coil heat-exchanger arrangements, the tubes are generally separated at an equidistant gap separation both vertically and horizontally.
The above-mentioned separation gap is an operational requirement to provide space between the tubes for ice buildup and to provide a path for fluid flow between the tubes and stored ice sleeves to recapture the stored cooling capacity. However, it is known that the uncontrolled growth of the ice on the tubes or circuits will, or may, result in complete horizontal bridging of the ice formed on the adjacent tubes. Although the total amount of ice stored may be sufficient for the application, the available thermal storage cooling capacity may be inadequate because only the perimeter of the formed monolithic ice block within the thermal storage unit is accessible to contact the circulating coolant, such as water resulting in higher leaving temperatures.
As a method to enhance recovery of the stored energy or cooling capacity, air agitation is typically provided at the bottom of the ice-storage chamber. This air travels upward through the gaps between adjacent tubes and ice masses. However, the development of monolithic or solid ice masses removes the separation gaps between adjacent tubes and the ice thereon, which inhibits air flow and fluid flow through the ice mass. The resultant effect is the reduction of the cooling capacity recovery as it is limited to the outer surfaces of the ice mass, which produces cooling fluid at higher and less useable temperatures. Attempts to improve efficiency at times utilizes extreme measures to melt the ice mass, such as using-high pressure hoses to melt the ice.
Although there are some monitoring techniques and equipment available to measure the volume of ice developed in a given chamber, it is a more general practice to visually inspect the tank volume. Another method utilizes a fluid level monitor based on the change of volume for ice, but these devices are not relied upon especially for shallow-volume tanks involving very small fluid-height changes. Therefore, overbuild ice conditions with monolithic ice blocks are a common and recurrent condition.
Consequently, it is desired to provide a means or method for greater access to more of the stored ice surface than just the outer perimeter of a monolithic ice block when an overbuild occurs.